Legionnaires' disease is endemic in Columbus with more than 70 cases identified since the summer of 1977. We have studied humoral immunity in these patients with the indirect fluorescent antibody and found that antibody does not persist at high titers for a long period. We also have demonstrated that lymphocytes from Legionnaires' disease cases demonstrate sensitization to antigens of Legionella pneumophila in the lymphocyte transformation assay. This reactivity appears to persist for a longer period than the antibody titers. We have also demonstrated that many subjects without a history of Legionnaires' disease manifest low titer antibody levels and reactivity in lymphocyte tissue cultures to L. pneumophila although at a lower level than patients who had documented Legionnaires' disease. Prevalence of immunity to L. pneumophila is unknown. Attack rates in epidemics have been reported to be 1-5%. We propose to expand our initial studies to determine the natural history of sensitization to L. pneumophila. We hope to be able to define an age range when subjects become sensitized to L. pneumophila. We also will look at the microepidemiology of subjects exposed to the hospital environment and the home environment of patients who develop Legionnaires' disease. To facilitate diagnosis we will determine the sensitivity and specificity of the enzyme-linked immunoassay to detect antigens of L. pneumophila in body fluids of patients with Legionnaires' disease.